1. Field of the Invention
The field relates to fabrication techniques and microelectromechanical systems (MEMS) and in particular to a methodology for fabricating MEMS systems on contour surfaces, such as cylinders.
2. Description of the Prior Art
Micron-sized mechanical structures cofabricated with electrical circuitry using conventional integrated circuit (IC) methodologies are called microelectromechanical systems or MEMS. In the past decade MEMS pressure sensors and accelerometers have realized commercial success due to the competitiveness in performance and cost in comparison to their conventional counterparts. Recently, a strong interest has risen in interfacing MEMS systems with macromechanical structures. The proposed application for integrating MEMS sensor networks into macromechanical parts include surface flow monitoring and control, condition based maintenance, environmental monitoring, process control, robotics and automation.
The possibility of controlling macromechanical components using MEMS actuators was demonstrated in 1994 by Ho et al., Control of Macro Machine by Micro Actuators, Bulletin of 47.sup.th Annual Meeting of the Division of Fluid Dynamics of the American Physical Society, Atlanta, Ga. (November 1994). In a wind tunnel experiment demonstrated by Ho, drag reduction was achieved by using MEMS actuators to reduce shear stress within the boundary layer of a flat plate. The University of Utah and Sarcos Research Corporation are currently developing packaging methods for MEMS pressure and vibration sensors combined with integrated circuits for underwater operation. The integrated sensor networks are to be implemented on a 1/8 scale version of a submarine fin. The Goodyear Tire & Rubber Company is also packaging MEMS sensors with radiofrequency transponders into truck tires for pressure sensing and revolution counting. To reduce payload weight, the satellite industry is also in need of a technology to incorporate integrated circuits with spacecraft mechanical structures.
Since IC process technology allows fabrication only on flat substrates, the MEMS structures must also reside on a flat chip. Thus, integration of these chips onto macromechanical components, which often have curved surfaces, becomes an issue which must be addressed. Typically, the bulk components are machined to accommodate the MEMS chips. The chips are then connected electronically by gold wires to complete the system. Other methods include for example fitting a completely hermetically sealed sensor and circuit system onto the machined parts. Thus the packaging of chips or systems onto mechanical components often change the contour of the bulk parts and this in turn requires precision machining.
A possible solution for packaging IC/MEMS devices onto macromechanical parts is to first fabricate the MEMS devices on a flexible skin, and then glue these skins onto mechanical parts. Obviously, these skins can be used to incorporate IC/MEMS devices onto conformal substrates without the need of developing a new technology. In fact, MEMS shear stress sensors have been built on lift-off polyimide films to alleviate the problem of machining the bulk parts. But MEMS and/or integrated circuits on flexible substrates are subject to some important limitations. One limitation is that the alignment of these skins to a desired orientation on the macromechanical component is in general a problem, which prevents low cost mass manufacturing. Another disadvantage is that this technique allows only the integration of surface micromachined devices onto the macrosubstrates, and does not allow the potential to bulk-micromachine a contour macrosubstrate. Also potentially interfering stresses caused by the glue and glue nonuniformity arise when attaching of these flexible skins onto a macromechanical substrate.
The patterning of simple, one-layer, micron-sized structures on cylinders has been performed by Ogura et al., "A Concentric Build-Up to Fabricate Practical Wobble Motors," IEEE 1994, 0-7803-1833-1/94 and Jacobsen, et al., "Fabrication of Micro-Structures Using Non-Planar Lithography," MEMS1993. Jacobsen patterned a 500 mm diameter rod with E-beam resulting in 10 micron line width resolution. Ogura did not disclose his patterning technique, but the rod was 1.0 mm in diameter with 30 micron line width resolution. Neither Jacobsen or Ogura disclose the procedure for aligning patterns of different layers of structures, which is critical for many integrated circuits and MEMS devices, such as is the case when fabricating transistors and the majority of existing MEMS devices.
A computer controllable laser system has been used to add silicon and deposit metals on the contour surfaces. Bloomstein and Ehrlich have developed a technique that deposit metals at approximately the rate of 2.times.10.sup.4 pixels per second with 1 micron resolution. See for example Bloomstein et al., "Laser-Chemical Three-Dimensional Writing for Microelectromechanics and Application to Standard-Cell Microfluidics," J. Vac. Sci. Technology, B 10(6) Nov./Dec. 1992. Metal thicknesses greater than 10 micron are obtainable and line widths as small 0.2 microns are possible.
Revise, Inc. is currently marketing an apparatus based on Bloomstein and Ehrlich's work under the trademark, SiliconEditor, which can deposit and etch serially different metals, semiconductors and dielectric materials through laser deposition techniques. The materials include aluminum, platinum, tungsten, silicon, molybdenum, gold and copper. The SiliconEditor also accepts data from computer aided design software to control stage motions and laser operation. The methodology is similar to that described by Jacobsen and is similarly expensive and slow, which ill adapts it to mass fabrication.
Therefore, what is needed is a methodology that will allow the multilayered fabrication of complex IC/MEMS devices directly onto nonsilicon contour substrates. Specifically, what is needed is a methodology for using thin film deposition, photolithography, and alignment of the mask layers for nonsilicon cylindrical substrates for the purposes of multilayer structures.